BUG/MEDIUM: cli: Don't read a command or payload line into a full buffer

BUG/MEDIUM: cli: Don't read a command or payload line into a full buffer

Commit e3c175871 ("BUG/MEDIUM: cli: Don't parse next command if input buffer
is empty") fixed a crash triggered when a command line exactly filling the
buffer was finished by a payload pattern, by no longer trying to parse a
next command when the input buffer was empty.

However this only addressed the case where no more data was available. When
a command line exactly fills the buffer and ends with a payload pattern, the
payload buffer is created with a size of 0 (no room left). If payload data
is then received on a subsequent read, the loop was not interrupted and
b_getline() was called with a length of 'b_room(buf) - 1'. Because b_room()
returns a size_t, an empty buffer made this length wrap around to SIZE_MAX,
and the line was copied past the end of the trash chunk, leading to a buffer
overflow.

To fix the issue, no attempt to read a command or a payload line is
performed when there is no room left in the destination buffer. In that
case, we fall back to the existing path that reallocates a bigger (dynamic)
payload buffer, which is guaranteed to be given a non-zero size, or reports
an error for the command line. This way b_getdelim() and b_getline() are
never called on a full buffer and the size_t underflow can no longer happen.

On older versions, 1-byte OOB write is also possible, but with a lower
severity.

This patch must be backported to 3.4, along with commit e3c175871. And it
should be adapted and backported to all other stable versions.